This application is based upon and claims priority from prior French Patent Application No. 99-03238, filed Mar. 16, 1999, the entire disclosure of which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to capacitive charge pumps, and more specifically to capacitive charge pumps that operate with a low supply voltage and methods for controlling the same.
2. Description of Related Art
A charge pump is a device for generating a voltage that is greater than a given supply voltage. Charge pumps are often, but not exclusively, used to control the gate of a floating-source power transistor that supplies a load (for example, an electric motor). A conventional xe2x80x9ccapacitivexe2x80x9d charge pump includes two different capacitors with the first injecting charges into the second, diodes that allow current to flow only in one direction, and an oscillator. Such a configuration makes it possible to obtain a voltage substantially equal to twice the supply voltage across the terminals of the second capacitor of the pump in the final charging state of the pump.
In practice, most charge pumps are xe2x80x9cfloatingxe2x80x9d and follow the supply voltage. Furthermore, practical charge pumps are typically regulated because they are often produced in CMOS logic. More specifically, they are based on complementary field-effect transistors with insulated gate that are limited in voltage (typically to 5 or 12 volts), while the supply voltage can exceed 12 volts (for example, when it is delivered by a properly charged battery of a vehicle). When the supply voltage is greater than the regulating voltage, the voltage delivered by the charge pump is reduced by an average voltage drop which depends on the current consumed at the output of the pump, the frequency of the oscillator, and the capacitance of the first capacitor of the pump.
At low supply voltage (i.e., when the supply voltage is less than the regulating voltage), the voltage delivered at the output of the charge pump is also reduced by the average voltage drop mentioned above, and by an auxiliary voltage term depending on the characteristics of the voltage regulator, in particular the saturation voltage of the associated current source. In order to compensate for these voltage losses in the voltage delivered at the output of the charge pump, and particularly at low supply voltages, there have been proposed solutions that aim to reduce the average voltage drop mentioned above. These solutions have been directed to increasing the capacitance of the first capacitor and/or the value of the frequency of the oscillator. However, such solutions are unsatisfactory, especially in terms of size and in terms of current consumption.
In view of these drawbacks, it is an object of the present invention to overcome the above-mentioned drawbacks and to reduce the charge pump voltage loss occurring at low supply voltage.
Another object of the present invention is to reduce the charge pump voltage loss at low supply voltages by focusing on the auxiliary voltage term rather than the average voltage drop.
One embodiment of the present invention provides a method for controlling a capacitive charge pump that is connected to a supply voltage. According to the method, the charge pump is regulated by a regulating voltage when the supply voltage is greater than the regulating voltage. When the supply voltage is less than a triggering voltage, which is less than or equal to the regulating voltage, the charge pump is automatically supplied between the supply voltage and ground. Thus, the auxiliary voltage term is reduced or eliminated by short-circuiting one of the supply terminals of the charge pump to ground. In a preferred method, the charge pump has a first supply terminal connected to the supply voltage and a second supply terminal that is automatically grounded when the supply voltage is less than the triggering voltage.
Another embodiment of the present invention provides a capacitive charge pump device that includes a charge pump having first and second supply terminals, a voltage regulator delivering a regulating voltage, a switch connected between the second supply terminal and ground, and switch control circuitry for automatically controlling the switch. The first supply terminal is connected to a supply voltage and the voltage regulator is connected between the first and second supply terminals. The switch control circuitry opens the switch when the supply voltage is greater than or equal to a triggering voltage, which is less than the regulating voltage, and closes the switch when the supply voltage is less than the triggering voltage. In one preferred embodiment, the switch includes an insulated-gate field-effect transistor whose drain-source voltage difference is low when the transistor is in an on state.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration only and various modifications may naturally be performed without deviating from the present invention.